FM is a way of sending radio sound. A station sends out a radio wave, then changes how fast that wave shakes — a tiny bit faster, a tiny bit slower — to carry the music or the voice. That system is called FM, short for frequency modulation. Frequency is how fast something shakes; modulation is a learned word for “changing it to carry a signal”.

AM is the older everyday sister-system: it changes the height of the wave instead. FM usually sounds cleaner, can be in stereo, and picks up less crackle from electrical noise. The trade-off is range: FM does not travel as far, and hills and buildings get in the way more easily.

In ordinary talk nobody says “frequency modulation”. People say FM radio, or they just read the number off the dial: “FM 91.7”. In Taiwan the letters F-M are what DJs say all day. Though actually, those two letters can stand for other things in other jobs; this entry is the radio one.